Author Archive: Gail

My company’s internal conference is in a couple of weeks, so this seems like a good time to have a quick rant about some presentation failings I’ve seen over the last year or so.

If you want to, or are planning to present at a conference (or even just a usergroup), please, please, please pay attention to the following.

Don’t read your presentation

Please don’t read the bullets on your slides one by one. Please also don’t read a speech off your phone. If I wanted to have something read to me, I’d get an audio book.

A presentation should feel dynamic. It is, and should feel like, a live performance.

If you need reminders or cue cards, that’s fine, but put keywords on them, points that need to be discussed, not the entire speech

Watch your font size

This is for the slides but especially for the demos. Font size of 30 is probably the smallest you should be using on slides.

In demos, if I’m sitting in the back row and can’t read the code, there may be a problem. My eyes are not the best though, so that might be a failing on my part. If, however, I’m sitting in the second row and can’t read the code, there’s definitely a problem.

If the conference insists on, or offers time for a tech check, take the opportunity to check your fonts. A tech check isn’t just ‘does my laptop see the projector? Yes, done.’ Walk to the back of the room, go through all the slides, start your demo, walk back to the back of the room. Make sure that everything is clearly visible.

Minimalistic slides

It’s distracting, and it probably means your audience is watching your slides and not listening to you. You should be the star of the presentation, not your slides. They’re a support character.

Themes

I like the Visual Studio dark theme. It’s nice to code with, it’s absolutely terrible on a projector. Especially if the room is not dark. For projectors you want strong contrast. Dark font on light background usually works. Dark blue on black does not, two similar shades of blue doesn’t.

Check that your demos are visible, check that the code is readable from the back of the room.

Learn how to zoom in, whether with the windows built in tools or installed apps. Use the zoom any time that what you’re showing may not be clear.

Repeat the question

Especially if the session is being recorded. Your voice is being recorded, the audience isn’t. It is so frustrating to listen to a recorded session, hear a minute of silence followed by the presenter giving a single word answer.

Even if the session is not being recorded, acoustics often make it possible for the presenter to hear a question while part of the audience hasn’t.

It also gives you a chance to confirm that you heard the question correctly and gives you a few moments to think on an answer.

The new adaptive query processing features in SQL Server 2017 are useful for fixing performance problems that were previously very hard to fix. They’re not perfect though, and one of the problems with memory grant feedback in particular is that it’s sensitive to data skew.

Before I get into why, let’s look at what adaptive memory grant does in the first place.

Queries request memory for operations like sorts, hash joins, hash aggregates and a few other operators. This is not TempDB space (ideally), it’s just memory. The amount requested is based on the optimiser’s guesses as to the size of the data that will be hashed/sorted, and that’s based off statistics and parameter values. Hence, there’s a chance for the guess to be wrong, and when it is, we get things like this:

When spill happen, the intermediate resultsets (or parts of them) do get written to TempDB. And read back. And potentially written and read back again, and maybe a few more times. This can be horribly slow.

Of course, there’s a chance that the estimate will be wrong in the other direction. Too large. It’s not as obviously bad, but it can limit the throughput of the system. Instead of the query running really slowly, it may have to wait before it runs at all, waiting for the memory to be granted. (RESOURCE_SEMAPHORE).

These were really hard problems to fix. There isn’t a query hint to request more or less memory than the estimates would allocate (though you can specify, as a percentage of the resource pool, the max and min memory to be allocated), so fixes had to be creative, typically tricking the optimiser into thinking there were more or fewer rows than there really were, or that the rows were wider (there are some lovely tricks that can be done with CROSS APPLY for example)

Adaptive memory grants don’t do anything to correct the optimiser’s mis-estimates. What they do, is allow the query processor to learn from the mistakes. If a query’s memory grant is significantly over or under what is needed, then a note is made of that, somewhere in memory, and the next time the query runs, the memory grant is adjusted to a value based on what the previous execution needed.

So, if we run the example from above a second time, making absolutely no changes in the process, the spills are gone.

This is great, unless you have a particular pattern in your workload, where one query will sometimes have a small number of rows flowing through it, and sometimes a large number. This is not a problem specific to Memory Grant Feedback. It’s been around for a long time, we call it bad parameter sniffing in many cases.

So let’s try a test of running the same query multiple times, alternating between parameter values that return small row counts and parameter values that return large row counts. The plan is the same in all cases, it’s a reused cached plan, and it’s one that’s not bad for the larger row counts (hash join, hash aggregate), so we don’t have the typical bad parameter sniffing problem, but the memory grant will oscillate, being based upon the previous query’s execution. I’m going to execute the stored procedure 200 times.

And I should mention that this is an extreme case. I specifically constructed a scenario where the memory grant required by one execution would be completely inappropriate for the next one. This is not (I hope) something that would happen in the real world.

I monitored what was happening with Extended Events, with the memory_grant_updated_by_feedback and memory_grant_feedback_loop_disabled events.

The results were kinda as expected.

And then something interesting happened. I didn’t clear the cache or anything, this was as the procedures executed in a loop.

After 8 executions, each with a memory grant update, both the execution count and the count of updates to the grant reset to 1.

This happened again 8 executions later

And again 8 executions later

Then, finally, after 32 executions, the update is disabled.

The procedure then went on to execute a further 168 times, with the same memory grant each time, equal to the last updated value.

So what can we conclude from this?

Firstly, there seems to be a re-evaluation of the memory grant feedback process every 8 modifications, deciding whether to continue adjusting. Second, it will stop adjusting memory grants at some point, though the conditions aren’t documented and I can’t tell from the test I ran what the conditions are. Since they’re not documented, they will probably change in future CUs/versions without notice.

Once the feedback cycle stops, the last memory grant value is what will be used for that query until its plan is removed from cache, at which point the adjustment cycle starts over from scratch.

If you’re working with a system that has this kind of query, with wide differences in optimal memory grant, I would suggest not relying on memory grant feedback, and changing the code so that the grant needed is more constant. This may require splitting procedures up, optimise hints or other fixes for bad parameter sniffing.

I suggest that because the feedback works great for ‘dialling in’ a good value for needed memory grant, but not for cases where the optimal grant is constantly changing. The 200 executions above took 4 minutes total without memory grant feedback, but 12 minutes with memory grant feedback.

It’s a great solution when the original estimate doesn’t match what the query needs, but it’s sub-optimal for queries with constantly changing memory needs. Procedures with widely changing memory needs should be fixed with other methods, including but not limited to multiple procedures, dynamic SQL, plan forcing, or other query hints.

One of the uses for the Query Store, added in SQL 2016, is to force plans. Once forced, plans are supposed to remain unchanged, however there are cases where a forced plan will not be applied and a new plan will be generated.

Statistics changes, which are one of the things that usually cause recompiles, don’t disable a forced plan. It would be kinda weird if it did and against the point of a forced plan.

Schema changes are another matter.

Let’s look at a couple of cases.

First, schema changes that make the plan invalid, in other words, schema changes that affect something that the plan explicitly references. There aren’t that many schema changes that can make the plan invalid without making the query invalid as well, but there are a couple. Index changes, for example.

I want to test a few things:

An index change that won’t make the plan invalid (eg adding a column)

An index change that does make the plan invalid (eg removing a column that the query needs)

Renaming the index without changing its definition

Adding an index that would be better for the query than the one referenced by the forced plan.

Now, let’s make that index less useful, by removing a column that the query does need. There’s a key lookup in the plan, so there is a way for the column to be obtained, but it would change what columns come from each operator and where the filters are being done. Same plan shape, but different details.

We get a new plan. The forced one is invalid, because the index no longer allows for the seek predicates defined in the plan, and so the forcing is ignored and we get a new plan.

The query still runs without error, which is better than we’d have had using the old USE PLAN hint.

Once I revert the index back to its original definition, the forced plan starts being used again.

How about renaming the index? Since the plan references the index by name, this will probably also cause the plan forcing to fail.

And indeed it does.

One last test. I’m going to rename the index back to its old name, and then add a new one that’s better for the query than the index referenced in the forced plan.

And we’re still using the forced plan. The addition of a new index did not invalidate the existing plan, and hence the forced plan will still be used, even when there’s a better index.

This is the reason why I recommend using plan forcing only to fix stuff that’s broken in prod, and to find a solution without forced plans for the long term. It’s not always possible but where it is I’d prefer not to leave the plan forcing in place, because it does mean that new indexes are not considered. Plus, if the query store is ever cleared, the forced plan (along with the forcing) are gone.

I originally wrote about catch-all queries early in 2009, just as something that I’d seen several times in client code. It turned into the 3rd most popular post ever on my blog.

A lot’s changed since 2009. When I wrote the original post, most production servers were SQL 2005 or SQL 2000. SQL 2008 had been out less than a year and its fix for catch-all queries, the RECOMPILE hint, didn’t even work properly (it had an incorrect results bug in RTM, was pulled in SP1 and fixed in SP2)

As such, my feelings on how to solve the problem with catch-all queries has changed over the years.

Before I get to solutions, let’s start with the root cause of the problem with catch-all queries – plan caching and the need for plans to be safe for reuse.

Let’s take a sample query. I’ll use the same one I used in the original post.

There are two nonclustered indexes on the TransactionHistory table, one on ProductID, one on ReferenceOrderID and ReferenceLineID.

For the initial discussion, let’s just consider two of the clauses in the WHERE. I’ll leave the other two in the stored proc, but they won’t be used.

WHERE (ProductID = @Product Or @Product IS NULL)
AND (ReferenceOrderID = @OrderID OR @OrderID Is NULL)

We would expect, if the ProductID parameter is passed, to get a seek using the index on ProductID, if the ReferenceOrderID parameter is passed, to get a seek using the index on ReferenceOrderID, and if both are passed, then either an index intersection or a seek on one of the indexes, key lookup and secondary filter for the other, plus, in all cases, a key lookup to fetch the columns for the SELECT.

That’s not what we get (I cleared the plan cache before running each of these).

The expected indexes are used, but they’re used for scans not seeks. Why? Let’s just consider the second plan for a bit.

The index aren’t used for seeks, because plans must be safe for reuse. If a plan was generated with an index seek, seeking for ReferenceOrderID = @OrderID, and that plan was cached and reused later when @OrderID was NULL, we’d get incorrect results. ReferenceOrderID = NULL matches no records.

And so we have index scans with the full predicate (ReferenceOrderID = @OrderID OR @OrderID Is NULL) applied after the index is read.

This is not particularly efficient, as the properties on the index seek shows.

The entire index, all 113443 rows were read, to return a single row. Not ideal, but it’s far from the largest problem with this form of query.

The plan’s got an index scan on the index on ReferenceOrderID, and then a key lookup back to the clustered index. That key lookup has a secondary filter on it, (ProductID = @Product Or @Product IS NULL). The optimiser assumed that a small number of rows would be returned from the index seek on ReferenceOrderID (1.47 to be specific), and hence the key lookup would be cheap, but that’s not going to be the case if the plan is reused with a ProductID passed to it instead of a ReferenceOrderID.

Before we look at that, the performance characteristics for the procedure being called with the ReferenceOrderID parameter are:

The duration and CPU are both in microseconds, making this a very fast query, despite the index scan.

Now, without clearing the plan cache, I’m going to run the procedure with only the ProductID parameter passed.

CPU’s gone from an average of 8ms to around 120ms. Duration has gone from average around 6ms to about 125ms and reads have jumped from 271 (2 MB of data processed) to 340 597 (2.6 GB of data processed)

And this is for a table that has 113k records and a query that returned 4 rows.

The key lookup, which was fine when an OrderID was passed, is not fine when @OrderID is NULL and the index scan returns the entire table.

The plans that the optimiser has come up with for this query form aren’t stable. They’re safe for reuse, they have to be, but performance-wise they’re not stable.

But, maybe it’s just this form of query, there are other ways to write queries with multiple optional parameters.

These both give us full table scans, rather than the index scan/key lookup we saw earlier. That means their performance will be predictable and consistent no matter what parameter values are used. Consistently bad, but at least consistent.

It’s also worth noting that neither of these will return correct results if there are NULL values in the columns used in the WHERE clause (because NULL != NULL). Thanks to Hugo Kornelis (b | t) for pointing this out.

And then two more forms that were mentioned in comments on the original post, slightly more complicated:

These two give the same execution plans as the first form we looked at, index scan and key lookup.

Performance-wise, we’re got two different categories of query. We’ve got some queries where the execution plan contains an index scan on one or other index on the table (depending on parameters passed) and a key lookup, and others where the execution plan contains a table scan (clustered index scan) no matter what parameters are passed.

But how do they perform? To test that, I’m going to start with an empty plan cache and run each query form 10 times with just the OrderID being passed and then 10 times with just the ProductID being passed, and aggregate the results.

Procedure

Parameter

CPU (ms)

Duration (ms)

Reads

SearchHistory

OrderID

5.2

50

271

ProductID

123

173

340597

SearchHistory_Coalesce

OrderID

7.8

43

805

ProductID

9.4

45

805

SearchHistory_Case

OrderID

12.5

55

805

ProductID

7.8

60

804

SearchHistory_Case2

OrderID

10.5

48

272

ProductID

128

163

340597

SearchHistory_Complex

OrderID

7.8

40

272

ProductID

127

173

340597

The query forms that had the clustered index scan in the plan have consistent performance. On large tables it will be consistently bad, it is a full table scan, but it will at least be consistent.

The query form that had the key lookup have erratic performance, no real surprise there, key lookups don’t scale well and looking up every single row in the table is going to hurt. And note that if I ran the queries in the reverse order on an empty plan cache, the queries with the ProductID passed would be fast and the queries with the OrderID would be slow.

So how do we fix this?

When I first wrote about this problem 7 years ago, I recommended using dynamic SQL and discussed the dynamic SQL solution in detail. The dynamic SQL solution still works very well, it’s not my preferred solution any longer however.

What is, is the RECOMPILE hint.

Yes, it does cause increased CPU usage due to the recompiles (and I know I’m likely to get called irresponsible and worse for recommending it), but in *most* cases that won’t be a huge problem. And if it is, use dynamic SQL.

I recommend considering the RECOMPILE hint first because it’s faster to implement and far easier to read. Dynamic SQL is harder to debug because of the lack of syntax highlighting and the increased complexity of the code. In the last 4 years, I’ve only had one case where I went for the dynamic SQL solution for a catch-all query, and that was on a server that was already high on CPU, with a query that ran many times a second.

From SQL 2008 SP2/SQL 2008 R2 onwards, the recompile hint relaxes the requirement that the generated plan be safe for reuse, since it’s never going to be reused. This firstly means that the plans generated for the queries can be the optimal forms, index seeks rather than index scans, and secondly will be optimal for the parameter values passed.

And performance-wise?

Reads down, duration down and CPU down even though we’re recompiling the plan on every execution (though this is quite a simple query, so we shouldn’t expect a lot of CPU to generate the plan).

How about the other forms, do they also improve with the RECOMPILE hint added? As I did before, I’m going to run each 10 times and aggregate the results, that after adding the RECOMPILE hint to each.

Procedure

Parameter

CPU (ms)

Duration (ms)

Reads

SearchHistory

OrderID

0

1.3

28

ProductID

0

1.2

19

SearchHistory_Coalesce

OrderID

6.2

1.2

28

ProductID

3.2

1.2

19

SearchHistory_Case

OrderID

1.6

1.3

28

ProductID

0

1.2

19

SearchHistory_Case2

OrderID

7.8

15.6

232

ProductID

7.8

11.7

279

SearchHistory_Complex

OrderID

1.5

1.4

28

ProductID

0

1.2

19

What can we conclude from that?

One thing we note is that the second form of case statement has a higher CPU, duration and reads than any other. If we look at the plan, it’s still running as an index scan/key lookup, despite the recompile hint.

The second thing is that the more complex forms perform much the same as the simpler forms, we don’t gain anything by adding more complex predicates to ‘guide’ the optimiser.

Third, the coalesce form might use slightly more CPU than the other forms, but I’d need to test a lot more to say that conclusively. The numbers we’ve got are small enough that there might well be measuring errors comparable to the number itself.

Hence, when this query form is needed, stick to the simpler forms of the query, avoid adding unnecessary predicates to ‘help’ the optimiser. Test the query with NULLs in the filtered columns, make sure it works as intended.

Consider the RECOMPILE hint first, over dynamic SQL, to make it perform well. If the query has long compile times or runs very frequently, then use dynamic SQL, but don’t automatically discount the recompile hint for fear of the overhead. In many cases it’s not that bad.

I gave my introductory session on transactions at all three of the South African SQL Saturdays in 2016, as well as at SQL Saturday Oregon in October 2017, and something that came up in most of them was the ‘manual sequence’, the idea of using a column in a table to store a max value and using that in place of an identity column or sequence object.

To be clear, I don’t think this is a good idea. The identity column works well if a sequential series of numbers are needed. If the number sequence needs to that spans tables, then the sequence object is a good replacement.

But, there will always be some requirements that insist on gap-less sequences, or insist on not using identity (probably for ‘compatibility’ reasons), so let’s see how to do it properly.

To start, the common attempt (taken from a random Stack Overflow answer)

And it doesn’t work because the select statement takes a shared lock. Shared locks are shared, and so multiple sessions can read the same max value from the table, then write back that same value+1 to the table, either generating duplicate rows or primary key/unique constraint violations (hopefully the latter)

So how do we fix it?

One option is to wrap the two statement in a transaction and add the UPDLOCK hint to the select. This ensures that no one else will be able to read the same max value from the table, but depending on indexes it could also cause some blocking and resultant slow queries.

Another way is to make the insert (or update) and the select a single atomic operation, by returning the inserted (or updated) value from the insert (or update) statement. We can use the OUTPUT clause for this.

The locking hints are, unfortunately, necessary. I tried several variations with less restrictive hints and they either:
– Produced duplicates
– Deadlocked when the table was small
– Deadlocked all the time

None of which are desired, hence the use of an exclusive table lock to serialise access. Of course, the restrictive locks will make this slow under concurrent usage. An index on ManualID will help, a bit.

Now we can test both of those the same way we tested the first version. An easy way to see whether there are any duplicates is to check the count and the distinct count.

To reiterate something I said earlier, I do not recommend using this. Identity columns, with their gaps, are fine for the majority of cases, especially the use of them for artificial primary keys. Artificial keys, if used, are meaningless numbers that should not be exposed to users, and hence gaps should be irrelevant.

The need for a gap-less sequence, stored in the table, should be an exceptional one, not a common one.

A common problem when looking at execution plans is attributing too much meaning and value of the costs of operators.

The percentages shown for operators in query plans are based on costs generated by the query optimiser. They are not times, they are not CPU usage, they are not IO.

The bigger problem is that they can be completely incorrect.

Before digging into the why of incorrect percentages, let’s take a step back and look at why those costs exist.

The SQL query optimiser is a cost-based optimiser. It generates good plans by estimating costs for each query operator and then trying to minimise the total cost of the plan. These costs are based on estimated row counts and heuristics.

The costs we see in the query plan are these compilation time cost estimates. They’re compilation-time estimations, which means that they won’t change between one execution of a query using a particular plan and another query using the same plan, even if the parameter values are different, even if the row counts through the operators are different.

Since the estimations are partially based on those row counts, that means that any time the query runs with row counts different to what were estimated, the costs will be wrong.

Let’s look at a quick example of that.

There are no customers with an ID of 0, so the plan is generated with an estimation of one row being returned by the index seek, and one row looked up to the clustered index. Those are the only two operators that do any real work in that plan, and each is estimated to read and fetch just one row, so each gets an estimation of 50% of the cost of the entire query (0.0033 it be specific)

Run the same query with a different parameter value, plans are reused and so the costs are the same.

That parameter returns 28 rows, the index seek is probably much the same cost, because one row or 28 continuous rows aren’t that different in work needed. The key lookup is a different matter. It’s a single-row seek always, so to look up 28 rows it has to execute 28 times, and hence do 28 times the work. It’s definitely no longer 50% of the work of executing the query.

The costs still show 50%, because they were generated for the 0-row case and displayed here. They’re not run-time costs, they’re compile time, tied to the plan.

Another thing can make the cost estimations inaccurate, and that’s incorrect costing calculations by the optimiser. Scalar user-defined functions are the easiest example there.

The first query there, the one that’s apparently 15% of the cost of the batch, runs in 3.2 seconds. The second runs in 270 ms.

The optimiser gives scalar UDFs a very low cost (they have their own plans, with costs in them though) and so the costs for the rest of the query and batch are meaningless.

The costs in a plan may give some idea what’s going on, but they’re not always correct, and should not be obsessed over, especially not when the plan’s a simple one with only a couple of operators. After all, the cost percentages add to 100% (usually).

Right, beginning of a new year, so time to look at what I read in the last year and what of it I can recommend.

Please ignore that it’s almost February. I’m going to pretend that the year starts with February, that way I don’t have to think about where January went.

First thing to mention is a change of tracking method. The blog plugin I was using to track my books doesn’t work under HTTPS. It gives odd errors when adding or updating books. I wasn’t in the mood for debugging php, instead the entire tracking of books and reading dates has been moved to GoodReads (with a few hiccups along the way)

My company puts on a year end function every year. It’s at some resort or other, and the important thing for this post is that we’re not told the location in advance. We find out when we get there (by bus).

What we are told, about a month ahead of the event, is approximate distances from 3-4 locations. These are where the bus pickup sites are. The locations are:

Head Office

Near Clearwater Mall

Fourways

Centurion

The distances given aren’t correct. And, as a result, there’s usually several attempts by various people to figure out where the year end function will be in advance.

I thought I’d join in this year, using some machine learning on those distances.

Now, I should mention that this is a very poor use for ML. Mainly because of a lack of data. I should have hundreds of data points for a decent prediction. I have 2 or 3 data points, for 4 different locations. Still, it’s what I have to work with.

First, the starting data. The distances for this year are:

Clearwater mall: 63 KM

Centurion: 56 KM

Fourways: 43 KM

HQ: 20 KM

Cape Town: 1447 KM

I’m going to ignore Cape Town for training, as it only had a distance previously specified in 2015, and so I only have one piece of data.

Plotting this on a map makes it clear that the distances have been ‘massaged’ (I’m plotting ‘as the bird flies’, not driving distance for ease of plotting, I’ll use driving distances for the training)

Now to stick those into a linear regression and see if I can predict the error on this year’s measurements.

I need to mention that with so little data, the accuracy of the linear regression is going to be very low. I’m as likely to get the correct results from linear regression as I am to get correct results from rolling a couple of d20s.

That said, onwards to untrustworthy results.

Once the starting values are loaded into R, creating a simple model is as easy as

Near-future Scotland. The book starts with a bank robbery, and the suspects are a bunch of orcs and a dragon. The robbery occurred in a persistent, online world, and the police are a little out of their depth. It gets more complicated from there.

A beautifully written story of the land of Arbonne, land of troubadours and joglars and courtly love, worshipping a goddess and ruled by a Queen; and a land to the north where only the warrior god is worshipped and the king and high priest have sworn to conquer Arbonne.

The first story of the Commonwealth saga, a futuristic society where space travel is almost unknown as wormholes link the worlds of the commonwealth together, and where people can live forever thanks to memory implants and rejuvenation techniques.

It all starts when an astronomer observes a star disappearing, enveloped in an instant by some form of Dyson sphere.

Another completely different fantasy setting. For centuries the Divinities had ruled and protected the continent, their miracles feeding the people, protecting them, etc. Then on one day, the Divinities were killed and civilisation on the continent collapsed.

Almost 100 years later strange things with a divine feel to them are happening and must be investigated.

A book full of strange questions and well-researched answers, such as “What would happen if the Earth stopped spinning?” (Hint: Bad things would happen), or “What would happen if you tried to hit a baseball travelling at 90 percent of the speed of light?” (Hint: Bad things would happen).

‘SARGable’ is a weird term. It gets bandied around a lot when talking about indexes and whether queries can seek on indexes. The term’s an abbreviation, ‘SARG’ stands for Search ARGument, and it means that the predicate can be executed using an index seek.

Lovely. So a predicate must be SARGable to be able to use an index seek, and it must be able to use an index seek to be SARGable. A completely circular definition.

So what does it actually mean for a predicate to be SARGable? (and we’ll assume for this discussion that there are suitable indexes available)

The most general form for a predicate is <expression> <operator> <expression>. To be SARGable, a predicate must, on one side, have a column, not an expression on a column. So, <column> <operator> <expression>

SELECT * FROM Numbers
WHERE Number = 42;

SELECT * FROM Numbers
WHERE Number + 0 = 42;

SELECT * FROM Numbers
WHERE Number = 42 + 0;

Any1 function on a column will prevent an index seek from happening, even if the function would not change the column’s value or the way the operator is applied, as seen in the above case. Zero added to an integer doesn’t change the value of the column, but is still sufficient to prevent an index seek operation from happening.

While I haven’t yet found any production code where the predicate is of the form ‘Column + 0’ = @Value’, I have seen many cases where there are less obvious cases of functions on columns that do nothing other than to prevent index seeks.

UPPER(Column) = UPPER(@Variable) in a case-insensitive database is one of them, RTRIM(COLUMN) = @Variable is another. SQL ignores trailing spaces when comparing strings.

The other requirement for a predicate to be SARGable, for SQL Server at least, is that the column and expression are of the same data type or, if the data types differ, such that the expression will be implicitly converted to the data type of the column.

SELECT 1 FROM SomeTable
WHERE StringColumn = 0;

SELECT 1 FROM SomeTable
WHERE StringColumn = ‘0’;

There are some exceptions here. Comparing a DATE column to a DATETIME value would normally implicitly convert the column to DATETIME (more precise data type), but that doesn’t cause index scans. Neither does comparing an ascii column to a unicode string, at least in some collations.

In generally though, conversions should be explicit and decided on by the developer, not left up to what SQL server decides.

What about operators?

The majority are fine. Equality, Inequality, IN (with a list of values), IS NULL all allow index usage. EXIST and IN with a subquery are treated like joins, which may or may not use indexes depending on the join type chosen.

LIKE is a slight special case. Predicates with LIKE are only SARGable if the wildcard is not at the start of the string.

SELECT 1 FROM SomeStrings
WHERE ASCIIString LIKE 'A%'

SELECT 1 FROM SomeStrings
WHERE ASCIIString LIKE '%A'

There are blog posts that claim that adding NOT makes a predicate non-SARGable. In the general case that’s not true.

SELECT * FROM Numbers
WHERE NOT Number > 100;

SELECT * FROM Numbers
WHERE NOT Number <= 100;

SELECT * FROM Numbers
WHERE NOT Number = 137;

These index seeks are returning most of the table, but there’s nothing in the definition of ‘SARGable’ that requires small portions of the table to be returned.

That’s mostly that for SARGable in SQL Server. It’s mostly about having no functions on the column and no implicit conversions of the column.

(1) An explicit CAST of a DATE column to DATETIME still leaves the predicate SARGable. This is an exception that’s been specifically coded into the optimiser.